This invention relates to display devices, and especially to display devices in which a display is built up by the selective illumination of individual elements. Such a device may, for example, be used for the display of character information.
It has been proposed to use d.c. electroluminescent elements as the individual elements. The selected elements are illuminated by repeatedly scanning the display elements in sequence, energising those required once each scan. Such a method requires complicated and powerful drive circuitry and inposes an inherent limitation on the number of elements in the display if the brightness of a bit element is not to fall below acceptable limits.
A display device has also been proposed using a.c. electroluminescent elements arranged in groups. Each group may be selected by closing a switch to apply an alternating voltage to a further a.c. electroluminescent element which thus lights and lowers the resistance of photoconductors coupled in series with the elements of the group. The particular element is selected by closing a switch so that the voltage is applied across that element through the photoconductor in series with it, whose resistance has been lowered. The voltage is then enough to cause the element to start to emit light. The light lowers the resistance of a further photoconductor in series with the element to the point at which the alternating voltage, applied through this photoconductor, can maintain the emission. Hence in this device an element once selected will remain set.
Emission is stopped by removing the alternating supply voltage--that is, all the lit elements will cease to be lit.
Such an arrangement prevents part only of the information on the display being removed. For a character display, for instance, that prevents one character only being selectively erased and rewritten without also rewriting all the unchanging information.